In recognition of these and other problems, during the making of the present invention, initial experimental results indicated greater reproducibility and improvement in sample optical quality, i.e. lowering of the optical absorption coefficient over previously used techniques and reduction of strain and percipitants, leading to an observation of .beta. (the absorption coefficient)=0.0007 cm.sup.-1 in a 5 cm diameter, 1 cm thick disk. The procedure has been successful for more than two dozen samples, including ingots exceeding 5 cm diameter by 10 cm long. Average optical absorption coefficient values at 10.6 .mu.m have lowered to less than 0.002 cm.sup.-1. Further results using this thermal anneal procedure for several crystals show that no significant differences appear among the various starting materials used.
The thermal anneal which yielded these results essentially follows close to the dope line, for example, of 2.times.10.sup.17 indium atoms/cm.sup.3. However, the annealing can take place anywhere in the area between the stoichiometric line and the dope line later crossing the dope line into the high resistivity region at some temperature-pressure therein, or by completely staying in the high resistivity region.
In practice, the sample is cooled to approximately 700.degree. C. where this equilibrium is realized. At that point, the sample is rapidly cooled to room temperature. Below 700.degree. C. diffusion rates are slow and the rate of cooling is not critical. However, cooling to room temperatures provides essentially the same results. One other factor which is important is the deviation from stoichiometry of the as-grown ingot. When, for example, the ingot is slow-cooled across the stoichiometric line following the isobar p (pressure)=0.8 atm cadmium a longer soak time is required to restore the desired composition to actually slightly cadmium poor. This factor has shown itself to be important for both quench and slow-cool thermal annealing procedures.
If a crystal is slow cooled to nearly room temperature from the crystal growing furnace, it will have a higher concentration of cadmium than a crystal taken out of the furnace at a higher temperature. Crystals, which were grown and slow cooled, were compared to crystals which were taken out hot at 880.degree. C. These latter crystals were found to have a different composition than the slow cooled crystals. According to theory, the crystal withdrawn at the higher temperature have a lower concentration of cadmium than the slow cooled crystals. Also, if the crystals are removed from the furnace at a high temperature corresponding to a stoichiometric composition or on the tellurium side of the phase diagram, the cadmium concentration will be a minimum.
It is believed that the crystal growing process (slow cool versus quench) was the cause of the inconsistency, the crystals were heat treated at a high temperature in the high resistivity range for a long time to establish composition equilibrium before starting the slow cool process. Consequently, if excess cadmium caused the inconsistency, it can be reduced through thermal equilibrium according to the phase diagram. In addition the object was also to keep the tellurium concentration at a minimum. Consequently, it meant working near the characteristic line corresponding to the doping concentration. The latter was tried first since it is nearer the high resistivity range than the stoichiometric line. Experimental results indicated that the optical properties are a function of the composition.